The invention relates to the field of protein engineering and screening for therapeutically relevant peptide. More specifically, it relates to cell surface display of post-translationally modified heterologous proteins. Heterologous display of proteins or peptides on the surface of a micro-organism (e.g. bacteria) is a useful research tool and has been associated with a broad range of interesting applications. Linking the protein or peptide function to the encoding gene enables the selection and/or optimization of peptides with desired properties from large combinatorial libraries. Various display formats have been developed including, ribosome display, phage display, bacterial surface display, and yeast display. Phage display is probably the best known system.
One of the most interesting applications of cell surface display is the selection of high affinity ligands from large libraries to therapeutically interesting target molecules. Thus far, only linear peptides, disulfide-linked cyclic peptides, and peptides coupled to an organic core have been displayed. This has resulted in the identification of various useful peptides, including therapeutically effective (lead) peptides. However, the proteolytic susceptibility and instability of these peptides has been recognized as a major disadvantage for therapeutic applications. The post-translational introduction of a cyclic structure in peptides, for instance a thioether crosslink, could circumvent these stability problems. However, at present it is still difficult to efficiently introduce such structures into synthetic peptides, especially for large peptides.